In ATM technology, a stream of cells must be transmitted over a physical link to a receiver, where the cell stream is demultiplexed into a series of virtual channels. If only one link is used, for example, a DS1 link, the transmission rate is obviously limited by the physical bandwidth of the link. The ATM Forum has defined a standard, known as IMUX, inverse multiplexing, whereby a cell stream can be divided among several links so that the total bandwidth to the cell stream is the aggregate bandwidth of the links. In accordance with a defined protocol, the cells are transmitted in a round robin fashion over the several links and assembled into a single cell stream at the far end.
The device for performing this operation is an Imux transceiver. It may conveniently be incorporated on a single chip. The Imux transmitter takes a stream of cells, and transmits them over a series of physical links, typically E1 or T1 access lines, and to a receiver which generates a corresponding stream of cells at the far end.
Typically an Imux will have sixteen output ports capable of accommodating a corresponding number of links. Increasing the number of ports results in an increase in the complexity of the chip, which is not justified in all situations. It is desirable to have a standardized chip rather than customize the chip for different markets.
For example, a DS3 link includes 28 T1 links multiplexed together. In order to send cells over a DS3 link, they must first be divided among the 28 T1 channels, but it does not necessarily follow that the IMA group will be chosen from the first sixteen links. A conventional sixteen port Imux cannot accommodate the remaining twelve links, yet a device with 28 ports has greatly increased complexity and also has a more limited market.
An object of the invention is to alleviate these problems.